Conventional charged-particle-beam (CPB) microlithography systems (typically using an electron beam as an exemplary charged particle beam) suffer from low throughput (i.e., the number of production units, such as wafers, that can be processed per unit time). To increase throughput, CPB projection-exposure apparatus (e.g., electron-beam steppers, etc.) have been developed that are capable of transferring large portions of the pattern to the substrate in one exposure “shot.” In one such apparatus, termed a “divided-reticle” CPB-microlithography apparatus, the pattern is defined on a reticle divided into a large number of “subfields,” each defining a respective portion of the pattern. Modern divided-reticle CPB-microlithography apparatus are configured to expose subfields measuring about 250 μm square (as exposed on the substrate). Simulation studies have revealed that the distribution of beam blur over such a large area is uneven. Simulation studies also have revealed that, whenever the current of a charged particle beam is increased in order to increase the throughput of the microlithography apparatus, space-charge effects also produce an uneven distribution of beam blur across the subfield. As a result, it is necessary to measure the distribution of beam blur at various points of the subfield with extremely high accuracy and precision. Based on these measurements, appropriate corrective adjustments can be made to the beam (e.g., of focal point, astigmatism, magnification, rotation, etc.), allowing the imaging performance of the microlithography apparatus to be improved.
A conventional technique for measuring imaging performance is shown with reference to FIGS. 10–13. Referring first to FIG. 10, it is understood that an illumination-beam source and a reticle, although not shown, are located upstream of the components shown in the figure (i.e., above the plane of the page). The reticle is positioned in the “object plane”. The multiple beamlets EB depicted in FIG. 10 are small electron beams produced by transmission of the illumination beam through respective rectangular measurement marks located in a subfield of the reticle. Hence, the beamlets EB that have passed through the measurement marks have rectangular transverse profiles. The beamlets EB are incident on a plate 102 that defines “knife-edge” reference marks 103a, 103b. The plate 102 is disposed on a wafer stage, which is positioned in the plane where the transferred image is to be formed (i.e., the “image plane”). The reference marks 103a, 103b are typically rectangular in profile and are configured as respective through-holes defined by the plate 102. The reference marks 103a, 103b define respective “knife-edges” 101a, 101b on which the beamlets EB are incident. The reference mark 103a is used for measuring beam blur along the X-direction of the plate 102, and the reference mark 103b is used for measuring beam blur along the Y-direction. In FIG. 10, nine X-direction reference marks 103a and nine Y-direction reference marks 103b are shown.
The knife-edge 101a is shown in FIG. 11. A beam-limiting diaphragm 105b is disposed downstream of the knife-edge 101a. The diaphragm 105b is made of a sufficiently thick, conductive metal plate that absorbs electrons of an incident beamlet EB. The beam-limiting diaphragm 105b defines a beam-limiting aperture 105. An electron detector (sensor) 106 is disposed downstream of the beam-limiting aperture 105. As shown in FIG. 12, beam currents detected by the electron detector 106 are amplified by a pre-amplifier 107, converted to an output waveform by a differentiation circuit 108, and displayed on an oscilloscope 109 or analogous display.
As shown in FIG. 12, a beamlet EB is incident in a scanning manner over the knife-edge 101a and the reference mark 103a. As the beamlet EB is scanned in a direction indicated by the respective arrow (labeled “SCAN” and extending to the right in FIG. 12), electrons e1, which are transmitted through the reference mark 103a, and a portion of electrons e2, which are forward-scattered through the plate 102, are transmitted through the beam-limiting aperture 105. Thus, most of the forward-scattered electrons e2 are blocked by the beam-limiting diaphragm 105b, and most of the electrons transmitted through to the detector 106 are the non-scattered electrons e1. The distribution of beam blur (i.e., blur variation, or “Δblur”) in the subfield can be measured by sequentially performing this measurement method for each of the nine knife-edge reference marks shown in FIG. 10.
A second conventional technique for measuring imaging performance is shown with reference to FIGS. 13–14. In FIG. 13, beamlets EB, as described above, are incident on a plate 102′, which defines multiple reflective reference marks 103a′, 103b′. Each reference mark 103a′, 103b′ is made of a thin film of heavy metal (e.g., Ta, W, etc.). The reference mark 103a′ is used to measure beam blur along the X-direction of the plate 102′, and the reference mark 103b′ is used to measure beam blur along the Y-direction. In FIG. 13, nine X-direction reference marks 103a′ and nine Y-direction reference marks 103b′ are shown. FIG. 14 shows an electron detector 116 disposed above the reference marks 103a′, 103b′. The detector 116 detects electrons that are incident on and reflected by the reference marks 103a′, 103b′ as the beamlet EB is scanned across the reference marks. The distribution of blur in the subfield is measured by sequentially performing the measurement method described above for each of the nine reference marks shown in FIG. 13.
In both of the conventional imaging-performance measurement techniques described above, beam blur resulting at each of the reference marks is measured individually (i.e., one at a time). Therefore, in order to measure the distribution of beam blur across an entire subfield, a large amount of time is needed.